This happened to me..... I went back into the last level again after completing the game and when I got the 1st lollipop the last achievement for lollipops unlocked

THIS IS NOT TRUE!!!! I can tell you now I never went in the credits section and still got this achievement by doing what I said in the response above because I thought my maths was wrong. 24x3x5=360 so where was the other 3 lollipops Collecting a lollipop in the last level again made it pop for me.

Both cases appear to be true. You can either collect the additional 3 lollipops in the credits screen, in which they will actually add to your total shown in the world select screen....or you can just replay the last level of the game upon collecting the 360th lollipop and the achievement should unlock as well, though this is technically the glitchy way to unlock it.